This invention relates to diaphragms for semiconductor devices. More particularly, it involves a method of making thin diaphragms of an accurately controllable thickness for semiconductor pressure transducers.
Generally, prior art semiconductor pressure transducers can be produced by etching central portions of the back side of a wafer until enough of the wafer has been removed to provide a diaphragm of the desired thickness. In production, however, it has been found that the etch rate is often inconsistent as it depends upon various factors such as the strength of the etchant solution. For example, where the same etchant is used to etch a plurality of wafers one at a time, the strength of the etchant solution diminishes according to the number of wafers which it has etched. Therefore, one had to periodically remove the wafer from the etchant and measure the diaphragm to determine its thickness. This process is tedious and one can easily over-etch the wafer. Hence, uniform diaphragm thickness is extremely difficult to obtain, especially between devices processed on different wafers. While techniques have been developed to automatically stop the etch on reaching a layer of etch resistant material, they still rely upon the inconsistent etch rate to ultimately define the diaphragm thickness. Moreover, until now no practical method has been developed in which diaphragms of a thickness on the order of several microns or less can be reproduced accurately and consistently between different wafers during production.
Uniform diaphragm thickness would substantially reduce calibration of the individual devices. Such calibration has heretofore been needed because the pressure transducer response is inversely proportional to the square of the diaphragm thickness. If one could accurately control the thickness of the diaphragms, response of all the pressure transducers produced in production would be substantially equivalent. By providing extremely thin diaphragms of about several microns thick, the sensitivity of the pressure transducers can be greatly enhanced. Hence, the diameter of the diaphragm can be decreased for a given pressure range to be sensed. Therefore, more devices per wafer can be produced.